1. Field of the Invention
The present invention relates to an optical recording apparatus incorporating an acoustooptic deflector capable of generating a plurality of laser beams by means of an ultrasonic wave to transfer information at a high information transfer speed.
2. Description of the Prior Art
A light beam, particularly, a laser beam, i.e., a beam of coherent light wave, is an optimum medium for high-density information recording and is employed in practical digital information recording apparatus and video signal recording apparatus because a light beam can be focused on a very small spot and can be controlled at a high speed.
Ordinarily, such a recording apparatus employing a laser beam irradiates a photosensitive recording medium with a single laser beam emitted from a laser beam source while the laser beam is controlled in an on-off control mode according to information to be recorded.
Referring to FIG. 4 showing an acoustooptic deflector for controlling a laser beam, there are shown a medium 1 formed of tellurium oxide (TeO.sub.2) or a lithium-niobium oxide (LiNiO), an electrode 2 formed on one end of the medium 1, a sonic wave absorbing member 3, an incident laser beam 4 falling on the X-Y plane of the medium 1, the zero-order beam 5 of the incident laser beam 4, and a first-order beam 6 of the incident laser beam 4.
When an alternating signal of a frequency in the range of 40 MHz to 100 MHz is applied to the electrode 2, a density-modulated ultrasonic wave 7 is generated in the medium 1 as indicated by broken lines in FIG. 4. The density-modulated ultrasonic wave 7 changes the refractive index of the medium 1 to form a cubic diffraction grating. Accordingly, when a laser beam of a diameter sufficiently larger than the wavelength of the density-modulated ultrasonic wave 7 falls on the medium 1 in a direction substantially perpendicular to the direction of propagation of the density-modulated ultrasonic wave 7, the density-modulated ultrasonic wave is deflected by diffraction (anisotropic Bragg diffraction) to generate the first-order beam 6.
Generally, the anisotropic Bragg angle at which the first-order beam is generated is approximately in inverse proportion to the wavelength of the density-modulated ultrasonic wave 7 generated in the medium 1 by the alternating signal applied to the electrode 2, and the intensity of the first-order beam is proportional to the power of the alternating signal applied to the electrode 2 within a fixed range of the power. Accordingly, when an alternating signal of a sufficiently high power is applied to the electrode 2, the acoustooptic deflector is able to convert the zero-order beam into the first-order beam at a conversion efficiency in the range of 80% to 90%.
Thus, the output first-order beam of the acoustooptic deflector can be controlled according to information to be recorded by the on-off control of the alternating signal applied to the electrode 2 according to the information.
However, in writing the information on an optical recording medium by controlling the light beam by such an acoustooptic deflector, the transfer speed is limited by a recording speed at which one bit of information is recorded, and hence a high-density recording apparatus is not necessarily a recording apparatus capable of operating at a high transfer speed.
To enable recording at a high transfer speed, there is proposed an optical recording apparatus of a multibeam system employing a semiconductor laser capable of simultaneously generating a plurality of laser beams and capable of controlling the plurality of laser beams to irradiate a single optical recording medium simultaneously by the plurality of laser beams for recording information in the optical recording medium. However, this proposed optical recording apparatus of a multibeam system has a problem that intervals between bits are increased inevitably because the optical recording apparatus must be provided with an optical device for focusing or controlling the plurality of laser beams in a predetermined relation to obviate crosstalk. Moreover, the construction of the optical recording apparatus of a multibeam system is very complicated.
It may be possible to divide a single laser beam into a plurality of laser beams by means of a diffraction grating. However, the individual control of the plurality of laser beams is very difficult.